The present invention relates to a composite carbide powder which serves as a raw material of cemented carbide, namely, hard metal and, more particularly, to the composite carbide powder of a tungsten carbide (WC) series. In addition, the present invention further relates to a method of producing the composite carbide powder.
Heretofore, a composite carbide powder is used to produce a wide variety of tools, such as an end mill, a reamer, a shear blade, and the like, to shear or shape an object. As such a composite carbide powder, WC--Co based cemented carbide has been disclosed in Japanese Unexamined Patent Publications Nos. Hei 3-208811 (namely, 208811/1991) and Hei 5-147916 (namely, 147916/1993) both of which will be referred to as first and second references, respectively. As mentioned in both the first and the second references, WC powder which has a mean grain size of 0.5 .mu.m or less is at first prepared as a raw material. To the WC powder, 0.1 to 2.0% of V, 0.1 to 2% of Cr, and 0.2 to 3% of Ta are added by weight as an inhibiter of grain growth. In addition, the cemented carbide further comprises, by weight, 5 to 30% of Co or Ni and impurities and has a structure specified by a WC phase of 0.8 .mu.m or less.
Recent demand has been strongly directed to an automatic shear process or an automatic shaping process. In such an automatic shear or shaping process, tools of cemented carbide have been exposed to very strict conditions. In order to endure such strict conditions, it is thought that the tools should be formed by fine-grain cemented carbide.
However, most of fine grain WC powders are practically liable to be easily carburized at a low temperature of 1300.degree. C. or less and are therefore unstable. As a result, grain growth readily occurs during a heat treatment of the cemented carbide. This means that a metallographic structure of a WC phase in the cemented carbide is adversely affected by wide distribution of grain sizes. Therefore, large-size particles have a defect which results in a reduction of strength in the cemented carbide. This makes it difficult to fulfill the requirements for the tools.
In order to produce WC powder with a grain size of 1 .mu.m or less, W powder should also have a fine grain size of 1 .mu.m or less.
Such W powder of a fine grain size is very expensive in comparison with W powder of 1 .mu.m or more and makes the cemented carbide expensive.